One may view the effector tissue for essential hypertension as the vascular walls of the small muscular arteries and arterioles. However, the contractile cells and intercellular contractile protein organization must be considered also as possible targets of the factor(s) which initiates and maintains the high blood pressure and increased peripheral resistance. We propose a quantitative investigation of the interaction of structure and function at several important levels of vascular control in the SHR and WKY rat - from the ultrastructural details of the smooth muscle cell to the responsiveness of the intact vessel in the living animal. We have provided fundamental information on the mechanical properties and contractile responsiveness of 75-300 micron m resistance vessels and of the cells within the walls of these vessels. The comparative tension-internal circumference properties of these vessels in 2 week old SHR and WKY rats has also been completed. These experiments were performed using a unique scheme for testing intact cylindrical segments mounted on wires which enabled us to visualize the cells. We propose to make further comparative measurements in 5 week and 42 week old SHR and WKYs, and to quantify the effects that anti-hypertensive treatment has on vessel and cell properties. Our in vitro wire-mount data will be compared with in vitro, pressurized vessel experiments where activated and relaxed pressure-diameter relationships of the same vessel will be made. Using micropressure measurements we will be able to define these characteristics in arterioles to 25 micron m. Hemodynamic and pressure-diameter characteristics of this bed will be also studied under flow conditions in the anesthetized rat down to the arteriolar level. This coordinated study of a single resistance bed from a single animal model of hypertension will provide new information and insights into the nature of the diverse abnormalities affecting the cardiovascular system and have relevance to human essential hypertension.